Thermosetting resins are widely used in a variety of electric and structural materials which are subject to casting, impregnation, lamination and molding. More rigorous requirements are now imposed on these materials in various applications. In particular, heat resistance and low moisture absorption are important factors for such materials.
Among prior art thermosetting resins, epoxy resins and phenolic resins were most useful. Known polyepoxy compounds for heat resistance purposes include phenol novolak epoxidized compounds (e.g., Epikote 154 commercially available from Yuka Shell Epoxy K. K.), cresol novolak epoxidized compounds (e.g., EOCN commercially available from Nippon Kayaku K. K.), methylenedianiline tetraepoxide, and epoxidized tri- and tetra(hydroxyphenyl)alkane compounds. Also known phenolic resins include phenol novolak resins, ortho-cresol novolak resins, resins of bisphenol-A and triphenolmethane, etc.
Cured products resulting from these resins exhibit heat resistance which is relatively high, but not fully satisfactory. Also, such resins undesirably require high temperature and long heating time in order to acquire a practically acceptable strength and are not satisfactorily easy to process. For semiconductor encapsulation purposes, not only are heat resistance, low moisture absorption and good processability required, but low coefficients of thermal expansion and firm adhesion are also needed.
In the prior art, thermosetting resin compositions comprising an epoxy resin, a phenolic resin as a curing agent for the epoxy resin, and an inorganic filler have been used for semiconductor packaging. A number of problems must be overcome in order to meet the recent requirement of thickness reduction of semiconductor packages. For example, when flat packages are mounted on printed wiring boards, the packages are immersed in a solder bath at elevated temperature. On solder immersion, cracks can occur in packages of conventional thermosetting resin compositions due to thermal shocks. If the flat packages have absorbed moisture before mounting on printed wiring boards, steam explosion can occur in the packages upon immersion in a hot solder bath, also inducing cracks.
Approaches for overcoming such problems are taken from both the frame and packaging resin aspects. As to the resin, improvements are needed in moisture absorption, coefficient of thermal expansion, and adhesion to frames as 7ell as mechanical strength and glass transition temperature.